This invention relates to a fusing device and a fusing method using a heating medium which has relatively low heat capacity.
Many reproducing devices, utilizes heat roll type fusing devices. The heat roll type fusing device comprises a heat roll and a pressure roll which form a fusing nip therebetween by pressing both rolls each other. A heating element is installed inside of the heating roll to heat a surface of the heat roll. A copy sheet is passed through the fusing nip to fix a toner image on the copy sheet. In heat roll-type fusing devices, the heat roller generally has a high heat capacity as a whole, requiring relatively long warm-up time, to heat the heat roll up to a sufficient temperature for fusing operation. In addition, high electric power is required to maintain the temperature of the heat roll surface at an appropriate temperature.
To avoid such problems, several fusing devices using a heating medium which has relatively low heat capacity are proposed.
The following disclosures about fusing devices using such heating medium may be relevant to the present invention.
JP-A 63-313182 discloses a fusing device comprising a heat-resistive film and a relatively small heating element. A copy sheet is contacted to the heat resistive film at a fusing nip. The heating element is installed at the nip portion on the backside of the heat resistive film to generate and apply heat energy to the toner image on the copy sheet in response to an input-pulsed signal. The copy sheet is detached from the heat-resistive film after the toner image is cooled and coagulated on the copy sheet. A heat-sink member may be introduced thereto during the cooling operation.
Japan Hardcopy '90, collection of articles published Jun. 20, 1990, pp. 53-56, titled as "New Fixing System `SURF` can reduce the warm up time" discloses a fusing device comprising a thin endless belt extended among two rollers including driving roller and a heater element. A pressure roller is mounted onto one side of the endless belt at a position where the heating element is contacted thereto from the backside. A copy sheet is passed through a nip formed between the endless belt and the pressure roller. A toner image on the copy sheet receives heating energy generated by the heating element from the backside thereof.
In these fusing devices, the film or the belt is typically heated at a fusing nip where the heating element contacts to the pressure member. Generally, such a pressure member have high heat capacity sufficient to reduce the heating efficiency of the heating element. The existence of such a member at the fusing nip still increases the required electric power for the fusing device and reduces process speed of the printing machine.
To enhance heat efficiency of the fusing device, several approaches have been proposed. Several fusing devices separating heat function from the pressure function have been reported to enhance the heating efficiency and reduce the electric power usage. The following disclosures describing separated functions of heat and pressure also may be relevant to the present invention.
U.S. Pat. No. 4,565,439 discloses a low mass heat and pressure fuser characterized by the separation of the heat and pressure functions such that the heat and pressure are effected at different locations on a thin flexible belt forming the toner contacting surface. A pressure roll cooperates with a non-rotating mandrel to form a nip through which the belt and copy substrate pass simultaneously. The belt is heated such that by the time it passes through the nip its temperature together with the applied pressure is sufficient for fusing the toner images passing therethrough.
U.S. Pat. No. 5,053,829 discloses a fusing apparatus including two nip forming members which cooperate to form a nip having an asymmetrical pressure profile. The pressure profile through the nip, from entrance to exit, is such that toner images on a substrate passing through the nip are first subjected to relatively low pressure which continues until the toner begins to flow. Once, toner flow commences, the images are subjected to pressure high enough to force the toner into the substrate. The nip is readily variable for accommodating different fusing speeds for different processors.
In those references, endless belts are used as a heating medium. The surface temperature of these heating medium is raised easily by applying relatively small amount of thermal energy. Also, the accumulated heating energy of the heating medium transfers from the endless belt to the toner image immediately. However, as these fusing devices release the toner image from the fusing nip right after the thermal transfer, the fused toner image sometimes remains on the surface of the heating elements. In other words, sometimes hot offset occurs onto the heating medium. This offset problem is typically induced when the fusing device is utilized for a fusing process of multi-layered color toner image. This is due to low melt viscosity upon melting of the color toner. In a color image fusing process, it is preferred that the fused toner images, including several different color-toners, should be melted and mixed completely each other. If any interfaces between toner layers remain in the fixed toner image, the brightness of the toner image will be lost because such interfaces will diffuse incident light into the toner image. It is also preferred that the surface of the fixed color toner image should have specific surface smoothness, typically more than 50% , preferably more than 80% in glossiness defined by JIS (Japanese Industrial Standard) Z8741-75. If the surface of the fixed toner image is not smooth, the color of the fixed image will be dimmed because the incident light will be reflected irregularly at the surface of the toner image rather than incorporated into the toner image. Relating to such problems, the following disclosures about fusing devices specialized as the color toner image also may be relevant to the present invention.
JP-A 4-372975 discloses a fusing device using a thin endless loop film. The thin endless loop film passes through several nip portions such a heat applying nip, a sheet detaching nip and a pressure applying nip formed by several roller members with a copy sheet. At the pressure applying nip, pressure is applied to a toner image on the copy sheet until the toner image is cooled and fixed onto the copy sheet, in order to mix color toners well to enhance color reproduction. The amount of pressure applied to the toner image and temperature of the toner image may be controlled at the pressure applying nip. The copy sheet is detached from the thin endless film at the detaching nip after the toner image is cooled and coagulated on the copy sheet.
JP-A 2-72376 discloses a fusing device using a thin endless film. First, heat energy is applied to a toner image on a copy sheet through the thin film, and then the toner image is kept under appropriate pressure by a pressure member while the copy sheet is transported with the thin film, sticking together until the toner image is cooled to a temperature below the softening point of the toner image. Then, the copy sheet is detached from the thin film. This operation controls surface smoothness of the fused toner image so that the image will have an appropriate surface glossiness.
JP-A 4-362679 discloses a belt-type fusing device comprising an endless belt, plural rollers on which the belt is extended, a fusing nip for applying a heat energy to a toner image on a copy sheet and a cooling roller removably mounted to a back surface of the endless belt. The amount of the contact area between the cooling roller and the backside of the belt is controlled in response to a requirement of surface glossiness of final toner image on the copy sheet. Finally, the copy sheet is detached from the surface of the endless belt after the cooling treatment.
U.S. Pat. No. 5,450,182 discloses a specific kind of belt fuser for fusing toner images on a transparency material. The toner images formed on the transparency during the imaging process have time to cool prior to separation from a smooth-surfaced belt. The peak fusing temperature is significantly higher than that used with conventional fusers such as heat and pressure roll fusers. This higher temperature guarantees excellent toner melting and flow thereby producing transparencies with excellent projection efficiency. The belt fuser is comprised of separated electrical sources and electrically resistive polyimide film.
However, in these fusing device, heating energy is still provided at the fusing nips, and relatively high fusing energy is still required to these fusing apparatus. Additionally, a relatively long cooling off time just after the fusing nip is required to the fusing device to avoid offset of the toner image. This relatively longer cooling off time contributes to slow process speed of the image reproducing apparatus.
The references cited herein are incorporated by reference for their teachings.